If you upgrade and sign for another 18months/2years, you are going to be in exactly the same position next June for the iPhone 5. If you have the money buy sim free and don't lock yourself into a contract. There are so many networks offering the iPhone now you would be stupid to stay loyal to any carrier for longer than 12 months.
Depends what other things you get from your network, I get 50% off my tariff from T-Mobile..... but I'm "Tied in" for 18 months, does that make me stupid?